A Pigeon hatches its young out in sixteen days, and by a special process and careful assiduity feeds them until they are almost full grown. A Fowl, though not capable of feeding her young in the same way, possesses the power of defending her offspring, finding and selecting suitable food, and attending to their education generally in a higher degree than the Duck can bestow on her progeny, which take four weeks to hatch. To still further illustrate this wonderful regulating principle, let us diverge for a moment from the eggs of birds to those of fish, where we find things adapted with incredible precision to the surrounding conditions of existence. I have noticed that trout living and being obliged to deposit their ova in a stream subject to great variations of temperature, spawn much earlier than trout in a stream preserving a comparatively even temperature. The condition of the latter in comparison with the former showed unmistakably that the difference of food supply did not account for it, and as the fry in both streams appeared about the same time in the spring, and exhibited no appreciable difference in size or strength during the summer, the natural conclusion to be arrived at is that the time required for hatching in each stream is contemplated and provided against in some mysterious way. =Mechanical Construction of Eggs.=--We now come to another phase of the protective principle, even more remarkable than those we have already discussed, and equally useful. This is in the mechanical construction of eggs to suit their situation and surrounding conditions. What an admirable provision Nature has shown in placing the axis of a bird's egg just where it will prevent it rolling off a flat surface, such as a ledge of rock, when moved by the terrible gusts of wind that sweep over high latitudes, or perhaps roughly moved by the parent bird suddenly fluttering off when scared. Take, for example, the egg of the Guillemot. This is so wonderfully constructed that if moved it will not roll away like a marble or billiard-ball, but simply spins round on its axis, in the same way as a screw or top, showing a wonderful adaptability to the exposed situation chosen by this bird for incubation. Birds which make round, cup-shaped nests or incubate in holes, such as the Owl and Kingfisher, for instance, lay round eggs, which run no risk of rolling away and being smashed. Their shape also facilitates alteration of position of the parent bird to secure an equal distribution of warmth and ventilation. Were the Guillemot and either of the latter birds to change nesting situations for a while, it is probable a speedy extermination of the species which adopted the flat rock for the round egg would soon take place, affording a beautiful illustration of the power that is also guiding the action of birds under the mysterious name of instinct. It is an unknown and unknowable power, yet its workings are as undeniable as its results.